CROSSING AND OLOSE-INTEEBREEDING. 239 



" The gain in constitutional vigor, derived from an 

 occasional cross, between individuals of the same va- 

 riety, but belonging to distinct families, or between 

 distinct varieties, has not been so largely, or so fre- 

 quently discussed as have the evil effects of close inter- 

 breeding. But the former point (the gain in constitu- 

 tional vigor, which is derived from crossing) is the 

 more important of the two, inasmuch as the evidence 

 is more decisive. The evil results from close-inter- 

 breeding, are difficult to detect, for they accumulate 

 slowly and differ much in degree, whilst the good 

 effects which almost invariably follow a cross, are from 

 the first manifest." 



In the following Chapters, we shall trace in detail, 

 the truth of the principle of Reversion in its application 

 to Crossing and Close-Interbreeding; and show that, 

 however widely the several species may differ in the 

 results of the several modes of Selection to which they 

 have been subjected, they all establish and confirm, 

 most positively, the theory that the evil results, of 

 Close-Interbreeding, are due to the absence, in the 

 individuals, of characters proper to their respective 

 species ; that the good results, from Crossing, are due 

 to each of the parents' contributing, to the offspring, a 

 character or characters which, are absent in the other 

 parent; that there is but one normal type, for each 

 species, which is the sum of all the positive characters 

 of such species; that no positive variation is possible, 

 in any species, after the members thereof, have re- 

 gained all the characters which the species once lost; 

 that any modification, or departure from the perfect 

 type of a species, is attended by physiological evil 

 which is merely aggravated by Close-Interbreeding; 



